The present invention relates to support structures for substrates in semiconductor processing chambers, and more particularly to low mass support structures for supporting wafers within a single-wafer processing chamber.
High-temperature ovens, called reactors, are used to create structures of very fine dimensions, such as integrated circuits on semiconductor wafers or other substrates. A circular substrate, typically a silicon wafer, is placed on a wafer support. Both the wafer and support are heated, typically by a plurality of radiant lamps placed around a quartz chamber. In a typical process, a reactant gas is passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of the reactant material on the wafer. Through subsequent processes, these layers are made into integrated circuits, with a single layer producing from tens to thousands of integrated circuits, depending on the size of the wafer and the complexity of the circuits. Other processes include sputter depositions, photolithography, dry etching, plasma processes. and high temperature anneals. Many of these processes require high temperature steps and can be performed in similar quartz reaction chambers.
If the deposited layer has the same crystallographic structure as the underlying silicon wafer, it is called an epitaxial layer. This is also sometimes called a monocrystalline layer because it has only one crystal structure.
Various process parameters must be carefully controlled to ensure the high quality of the resulting layers. One such critical parameter is the temperature of the wafer during the processing. During CVD, for example, the deposition gas reacts at particular temperatures and deposits on the wafer. If the temperature varies greatly across the surface of the wafer, uneven deposition of the reactant gas occurs.
In certain batch processors (i.e., reactors which process more than one wafer at a time) wafers are placed on a relatively large-mass susceptor made of graphite or other heat-absorbing material to help the temperature of the wafers remain uniform. In this context, a xe2x80x9clarge-massxe2x80x9d susceptor is one which has a large thermal mass relative to the wafer. The thermal mass of a solid, or its lumped thermal capacitance, is given by the equation:
CT=xcfx81Vc
where:
xcfx81=the density of the solid,
V=the volume of the solid, and
c=the specific heat (heat capacity) of the solid.
Thus, the thermal mass is directly related to its mass, which is equal to the density times volume and to its specific heat.
One example of a large-mass susceptor is shown in U.S. Pat. No. 4,496,609 issued to McNeilly, which discloses a CVD process wherein the wafers are placed directly on a relatively large-mass slab-like susceptor and maintained in intimate contact to permit a transfer of heat therebetween. The graphite susceptor supposedly acts as a heat xe2x80x9cflywheelxe2x80x9d which transfers heat to the wafer to maintain its temperature uniform. The goal is to reduce transient temperature variations around the wafer that would occur without the xe2x80x9cflywheelxe2x80x9d effect of the susceptor.
Although large-mass susceptors theoretically aid in maintaining temperature uniformity across the wafers when the system is in a steady state, the large thermal mass of the susceptor makes the susceptor-wafer combination slow in responding to temperature transients (e.g., while heating up or cooling down the system). Accordingly, processing wafers with large-mass susceptors involves long thermal cycles, limiting the number of wafers which can be processed in a given length of time (i.e., limiting process throughput). High throughput remains a prime concern in single-wafer semiconductor processing.
In recent years, single-wafer processing of larger diameter wafers has grown for a variety of reasons including greater precision process control as compared to batch-processing. Typical wafers are made of silicon with one common size having a diameter of 200 mm and a thickness of 0.725 mm. Recently, larger silicon wafers having a diameter of 300 mm and a thickness of 0.775 mm have been introduced, as they even more efficiently exploit the benefits of larger single-wafer processing. Additionally, even larger wafers are contemplated for the future.
Although single-wafer processing by itself provides advantages over batch processing, control of the process parameters remains critical and is perhaps more so because of the increased cost of the larger wafers. One example of a single-wafer processor is shown in U.S. Pat. No. 4,821,674, which utilizes a circular rotatable susceptor having a diameter slightly larger than the wafer. This susceptor is preferably made of graphite and has a lower thermal mass than the aforementioned slab-type batch processing susceptor. Nevertheless, the thermal mass of a production version of the susceptor described in U.S. Pat. No. 4,821,674 is larger than the thermal mass of the single wafer, such that thermal cycle time for the system is limited.
U.S. Pat. No. 4,978,567 describes a wafer holding fixture of lower mass than conventional susceptors. The lower mass facilitates rapid heating and cooling of the wafer for Rapid Thermal Processing (RTP) systems. Throughput can also be increased in connection with other processes involving heating or cooling of a substrate to be processed.
Processing wafers with such a low-mass wafer holder, however, introduces new problems. For example, the low mass of the wafer holder, combined with a small gap between the wafer and holder, makes it difficult to lift a wafer off the holder without also lifting the wafer holder. A vacuum effect causes the wafer and holder to stick together. As gas starts to fill the small gap, the gap will increase and the gas will flow faster. Accordingly, the holder will drop shortly after pick-up. Obviously such an uncontrolled drop can cause damage to the wafer holder and surrounding equipment within the reaction chamber. Additionally, particulate matter created by such damage can contaminate processed wafers.
The very rapidity of thermal response for which the low mass wafer holder is designed can also cause damage to the wafer and to reactor parts. For example, when first introduced into a reaction chamber, the wafer may be cold (e.g., 200xc2x0 C.), while the wafer holder remains hot (e.g., 900xc2x0 C.) from processing a prior wafer. Bringing the cold wafer into contact with a hot wafer holder causes a rapid heat drain from the holder to the wafer. The low mass wafer holder rapidly drops in temperature, as compared to the rate at which a high mass susceptor would drop, until the wafer and wafer holder are in thermal equilibrium. The wafer, in the interim, undergoes a rapid heat influx. The rapid temperature fluctuation causes thermal shock to both the wafer and the holder. Both the wafer and holder tend to bow under the strain of vertical and radial temperature gradients during the transition. The stress can often cause breakage of the wafer holder and, occasionally, even the wafer.
The lower mass wafer holder is also susceptible to thermal expansion during heating. Due to differences in equipment material, the wafer holder will tend to expand at a different rate, as compared to surrounding equipment. In particular, a structure for supporting and rotating the wafer holder during processing is often constructed of quartz, such that radiant heat from below will largely pass through this structure. A typical graphite or silicon carbide (SiC) wafer holder expands significantly more rapidly than the quartz structure.
Relative movement between the supporting quartz and the wafer holder due to differences in thermal expansion can cause decentering of the wafer holder and the wafer upon it. Decentering, in turn, can tilt the wafer holder or otherwise upset a carefully balanced relationship between reactor elements and the wafer, configured for achieving temperature uniformity. Furthermore, eccentricity will exacerbate the decentration, such that the wafer holder can come in contact with a slip ring or other adjacent structure, bumping or rubbing against these structures during rotation and potentially introducing particulate matter into the reactor. Decentering can thus cause non-uniformity in the quality and thickness of deposited layers, for instance.
Consequently, there is a need for an improved low mass wafer support structure to increase throughput of semiconductor processing devices while ensuring temperature uniformity across the wafer surface. Desirably, such a support structure should avoid the above-noted problems associated with wafer pick-up, thermal shock, and thermal expansion.
In accordance with one aspect of the present invention, a low mass wafer holder is provided for supporting a substrate within a process reactor. The holder has an upper surface and a lower surface, with a plurality of lips integral with the upper surface. Because the lips are integral, the lips can be machined to a uniform height above the upper surface, thereby supporting the substrate with a uniform gap between substrate and the upper surface. In one embodiment, the upper surface is discontinuous, including the top surface of a peripheral supporting ring, as well as the top surface of a central base plate.
In accordance with another aspect of the present invention, a wafer holder for is provided for supporting a substrate within a process reactor. The wafer holder includes a central portion with an upper surface and a plurality of spacers projecting a uniform height above the upper surface. The spacers are distributed to peripherally support the substrate above the upper surface. The holder also includes a fringe portion, including a ring inner wall extending upwardly from and surrounding the upper surface. The ring inner wall and the upper surface of the central portion thus together define a substrate pocket for accommodating the substrate. The peripheral location of the spacers minimizes any risk of thermal disturbance from the discrete spacer contact with the substrate being processed.
In accordance with another aspect of the present invention, a semiconductor reactor is provided for treating a substrate. The reactor includes a reaction chamber, a plurality of heat sources, and a self-centering single-wafer support structure. The support structure is self-centering in the sense that it is centered and level at a first temperature as well as at a second, different temperature. The support structure includes a wafer holder for directly supporting the substrate, characterized by a first coefficient of thermal expansion. At least one recess is formed in a bottom surface of the wafer holder. The support structure also includes a support spider for supporting the wafer holder, characterized by a second coefficient of thermal expansion different from the first coefficient. The spider includes at least three support posts cooperating with the recess of the wafer holder.
In one embodiment, the support posts cooperate with three radial grooves formed in the bottom of the wafer holder, distributed at 60xc2x0 intervals. In accordance with this embodiment, the wafer holder remains centered on the spider at any given temperature, despite differential thermal expansion of the holder relative to the spider. In another embodiment, each support post includes a hot-centering surface and a cold-centering surface. At high temperatures, the hot-centering surfaces of at least three support posts define a restrictive circle outside the peripheral edge of a base plate of the wafer holder. At low temperatures, the cold-centering surfaces of at least three support posts define a restrictive circle inside of the inner wall of a peripheral ring of the wafer holder.
In accordance with another aspect of the present invention, a low mass wafer holder is provided for supporting a single substrate in a processing chamber. The wafer holder includes a disc-shaped base plate and an annular ring independent of the base plate. The holder also includes an annular hanging portion integral with either the ring or the base plate, characterized by an inner diameter, and an annular supporting portion integral with the other of the ring and the base plate, characterized by an outer diameter smaller than the inner diameter of the hanging portion. The supporting portion underlies and supports the hanging portion. The two-piece wafer holder design has been found to alleviate stresses associated with thermal transfer for low mass wafer holders.
In accordance with another aspect of the present invention, a semiconductor reactor includes a reaction chamber, a plurality of heat sources, and a wafer support structure for supporting a wafer. The wafer support structure includes a low mass wafer holder, which directly supports the wafer. A temperature sensor, connected to at least one of the heat sources, senses the temperature at a point vertically spaced from the wafer holder. In one embodiment, a thermocouple is spaced below the wafer holder within a spacing range wherein the temperature reading is relatively insensitive to spacing changes. The wafer temperature can thereby be indirectly controlled at an appropriate temperature without regard to spacing changes caused by thermal expansion and other typical factors.
In accordance with another aspect of the present invention, a low mass wafer holder is provided for supporting a single substrate in a processing chamber. The wafer holder includes an upper support surface with a plurality of open radial channels. Each of the channels has a width less than the thermal diffusion length in the substrate (e.g., less than about 5 mm for a silicon substrate). The total volume of the channels is sufficient to permit lifting the substrate independently from the wafer holder. As the wafer is lifted, gas is permitted to flow beneath the wafer, such that no vacuum effect takes place and the wafer is easily separated from the wafer holder.
In accordance with another aspect of the present invention, a wafer holder is provided for supporting a substrate. The wafer holder has a thermal mass less than about five times the thermal mass of the substrate. The holder includes a base plate extending generally parallel with and spaced below the substrate. An annular lip peripherally supports the substrate above the base plate, and includes an inner face which defines a gap between the substrate and the base plate. A gas passage communicates from an underside of the wafer holder to the gap between the substrate and the base plate.